Injection-locked laser stabilizer

ABSTRACT

An injection-locked laser stabilizer is provided utilizing an injection-locked laser system comprising a small stable laser, an optical frequency isolator (circulator) and a high power laser which is injection-locked by the small stable laser. The stabilizer provides an automatic acquisition of the injectionlocked condition and maintains that condition for an indefinite period.

ilnited States Patent [1 1 [111 EJMMM Sasnett 1 July 17, 1973 [54]INJECTION-LOCKED LASER STABILIZER 3,633,123 1/1972 Marcatili 331/56 4,[75] Inventor: Michael W. Sasnett, Los Altos, Calif. 3 53 288 10/1970Forster 332/7 51 [73] Assignee: The United States of America as B "P MZlifiiflfifiii'iii'iifiifi13 A" Force washmgton Attor neyHarry A.Herbert, Jr. and George Fine [22] Filed: May 14, 1971 [21] Appl. No.:143,368

[57] ABSTRACT [52] Cl 330/43 331/56 331/945 An injection-locked laserstabilizer is provided utilizing 332/751 an injection-locked lasersystem comprising a small sta- [5 11 Ill. Cl. H018 ble laser an opticalfrequency isolator (circulator) and [58] Field of Search 332/751;330/43; 3 high power laser which is injectiomlocked by the 331/945 56small stable laser. The stabilizer provides an automatic acquisition ofthe injection-locked condition and main- [5 6] References cued tainsthat condition for an indefinite period.

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HRRNONIC) 0 INVENTOR. El IE 0 3 MICHEAL w. SASNETT BY flg/ WEJ ATTORNEYSBACKGROUND OF THE INVENTION This invention relates to injection-lockedlasers and more particularly an injectiornloclted laser system includinga stabilizer to maintain the laser in the injection-locked condition.

A technique referred to as laser injection-locking has previously beendemonstrated by I-I.L.Stover and W.M.Steier in the article entitledLocking of Laser Oscillators by Light Injection published in Appl. Phys.Lett. 8, pp 91-93, Feb. 15, 1966 and described by C.L. Tang and H.Statz,in Phase-Locking of Laser Oscillators by Injected Signal, Journal Appl.Phys. 38, pp 323-324, Jan. 1967 whereby the frequency of a high powerlaser oscillator may be locked to the signal frequency of a low power,and more stable, laser oscillator. The injection-locking technique haspreviously found wide application in the radio frequency range. Thepresent invention allows a laser oscillator to remain injection-lockedover an indefinite time period, and allows automatic acquisition of theinjectionlocked condition.

Previously, no method has been available for accomplishing either ofthese. For lasers, the injection-locked condition can only be maintainedfor very brief periods on the order of from l to seconds, unlesstechniques such as the one herein described are used.

SUMMARY OF THE INVENTION An injection-locked laser stabilizer of thepresent invention is comprised of an injection-locked laser system and astabilizer in combination therewith. The injection-locked laser systemis comprised of a small stable laser emitting a first beam, an opticalfrequency isolator (circulator) directing the first beam to a high powerlaser which is ultimately injection-locked. The high power laserprovides an output beam, by way of the aforesaid isolator and also thereflected first beam. The stabilization occurs since the high poweroscillator also includes as part thereof a cavity tuner. Then theresonant frequency of the high power laser cavity is modulated byapplying a sinusoidal signal to the cavity tuner. A detector receivesthe output and reflected beam. A signal due to power variation at thedetector appears. The amplitude, A, and. phase, (I), of this signalvaries. This error signal is used to electronically measure themagnitude and direction of offset between the high power cavityresonance frequency and the stable laser frequency. This then permitsthe utilization of an electronic feedback control loop which uses theerror signal to maintain the high power laser in the injectionlockedmode of operation. An additional optical path is provided in which partof the output of the stable laser is split off and directed through afrequency shifter. The frequency shifter is driven by an RF. oscillator.The frequency shifted beam is combined with the unshifted beams at thedetector producing a heterodyne bent signal at the detector which isutilized in a feedback path to the high power laser to automatically setits frequency at the frequency of the injected signal. This then allowsautomatic acquisition of the injectionlocked condition. There is thustwo important advantages that are provided by the invention. One, theinjection-locked condition is automatically acquired. Two, theinjection-locked condition is automatically maintained. This allows muchhigher output to be realized from an injection-locked oscillator system,and the requirements on the inherent frequency stability of the highpower oscillator are much reduced.

DESCRIPTION OF DRAWINGS FIG. I of the invention illustrates in blockdiagram form an injection-locked laser stabilizer.

FIG. 2 illustrates the power curve at the photodetector of FIG. 1; and

FIG. 3 illustrates curves of amplitude and phase at the photo-detectorof FIG. 1 due to audio oscillator 25.

DESCRIPTION OF PREFERRED EMBODIMENT Now referring in detail to the FIG.1 of the present invention, there is shown conventional low power stable laser 10 emitting beam 1. The symbol for beam 1 is A sin w t. Beam 1is directed by way of conventional optical beam splitter l l toconventional optical isolator (circulator) I2. Isolator l2 directs beam1 to conventional high power tunable laser 13.. Tunable laser 13includes a conventinal cavity tuner 13a. High power tunable laser 13provides output beam 2 and also reflected beam 1 to isolator 12 whichdirects them to mirror 16. The symbol for beam 2 is A sin al t Beam 1 isalso directed by way of beam splitter 11 to conventional mirror 14 whichthen reflects it to conventional frequency shifter 15. Frequency shifter15 is driven by RF. oscillator 26. Frequency shifter 15 then provides anan output beam 3 which is also directed to mirror 16. Beam 3 has thesymbol A sin w, w,,)t. Mirror 16 then directs reflected beam 1, outputbeam 2, and beam 3 to photodetector 17. An output beam for theinjection-locked stabilizer is also provided at 18 also by way of mirror16. The output from photodetector 17 is simultaneously fed through F.M.discriminator 19 and AC. amplifier 20. The output signal from F.M.discriminator I9 is passed through I.F. amplifier 21 to summationnetwork 23 and the output from A.C. amplifier 20 is fed through productdetector 22. Also to summation network 23. Amplifier 24- interconnectssummation network 23 and cavity tuner 13a. Cavity tuner 13a alsoreceives a signal from audio oscillator 25.

Now referring to the operation of the injectionlocked laser stabilizershown in the FIG. I, the optical power level at detector 17 depends onthe power (P) and the relative phase angle of beams l and 2 according tothe relations P k [2 wlP P cos (b P P Since the path length traveled byboth beams (the reflected beam and the output beam) leaving the outputof high power laser 13 is the same, the only phase difference which canarise willbe due to fluctuations in the resonant frequency of high powerlaser cavity 13a while it is being injection-locked. This phasedifference is given by d sin-l 1 O/ Iot'k] where w, is the frequency ofthe injected signal (beam 1) and (n is the resonant frequency of highpower laser cavity 13a. The power measured by detector 17 is thereforeat a maximum when m, w and d) 0. If the high power laser resonantfrequency drifts to one side or the other of the input signal frequency,a power reduction at detector 17 will result. P will have the form shownin FIG. 2.

When the resonant frequency of high power laser cavity 13a is modulatedby applying a sinusoidal signal from audio oscillator 25 to cavity tuner13a, a signal due to power variation at detector 17 will appear. Theamplitude, A, and phase, (11, (with respect to the audio oscillator) ofthis signal will vary as shown in FIG. 3. Only the fundamental harmonicof the signal is of interest.

This signal may then be used to electronically measure the magnitude anddirection of offset between the high power cavity resonance frequencyand the stable laser frequency. This then allows the use of anelectronic feedback control loop which uses this error sig nal tomaintain the high power laser in the injectionlocked mode of operationeven though its cavity resonance frequency may be inherently unstable.This technique makes use of an A.C. rather than a DC. error signal andthus avoids the problems involved with the stability of DC. signals.

By adding an additional optical path in which part of the output ofstable laser is split off and beam 3 is thus directed through frequencyshifter 15 the capability for automatic acquisition of the injectionlocked condition is realized. The frequency shifted beam is combinedwith the unshifted beams from high power laser 13 producing a heterodynebeat signal at detector 17. When this beat signal is just equal to thefrequency of R.F. oscillator 16 that drivesfrequency shifter 15, thenstable laser 10 and high power laser 13 will be operating at the samefrequency. Therefore, by choosing discriminator 19 to have a centerfrequency equal to the R.F. oscillator frequency an error signal isdeveloped which when amplified and fed back to high power laser13'automaticallysets its frequency at the frequency of the injectedsignal. This then allows automatic acquisition of the injection lockedcondition. The automatic phase control circuit then takes over tomaintain the injection-locked condition. Note that with the automaticfrequency control provision, reacquisition of the injection-lockedcondition is assured in the event that some fast transient conditioncauses the'high power laser to become unlocked.

The advantages are the injection-locked condition is automaticallyacquired and the injection-locked condition is automatically maintained.This allows much higher power output to be realized fromaninjectionlocked'oscillator system, and the requirements on the inherentfrequency stability of the high power oscillator are much reduced.

The ratio between the input power and output power levels (P, and P inan injection-locked system is sub ject to the requirement that if theresonant frequency of the high power laser cavityv drifts back and forthwithin a range 2A0) then,

2A") "o/Q i/ 21 where w oscillation frequency and Q cavity qualityfactor for high power laser. The gain available in an injection lockedsystem is then given by Since the injection-locked laser system veryeffectively reduces Am, the gain available from an injection lockedsystem is much larger than would otherwise be possible.

I claim:

1. An injection-locked laser stabilizer comprising a low power stablelaser providing a first beam at a predetermined frequency, a high powerlaser providing a second beam at a preselected frequency, said highpower laser having output means for said second beam and also includinga cavity tuner integrated with said high power laser, audio oscillatormeans providing a preselected frequency signal to modulate said cavitytuner, first means to pass said first beam in a first direction anda'portion thereof in a second direction, second means receiving saidpassed first beam for direction into said output means of said highpower laser for injection-locking purposes, said high powerlaserreflecting said first beam from said output means and alsoproviding therefrom said second beam to said second means,photo-detector means, third means receiving said reflected first beamand said second beam from said second means for direction to saidphoto-detector means, frequency shifting mea'ns, R.F. oscillator meansoperating at a preselected frequency for driving said frequency shiftingmeans, fourth means directing said portion of said first beam to saidfrequency shifting means for a preselected frequency shift thusproviding a third beam, said third beam also being directed to saidphoto-detector means by way of said third means, F.M. discriminatormeans, said F.M. discriminator having a center frequency equal to theR.F. oscillator frequency, A.C. amplifier means, said F.M. discriminatormeans and said A.C. amplifier means receiving simultaneously the outputsignal from said photo-detector means, a product detector receiving asignal from said A.C. amplfier means, a summation network receivingsignals from said F.M. discriminator means and said product detector toprovide a resultant signal for reception by said cavity tuner to providean automatically acquired injection-locked condition and toautomatically maintain the injection-locked condition in said high powerlaser.

2. An injection-locked laser stabilizer as described in claim 1 furtherincluding an LP. amplifier interconnecting said F.M. discriminator meansand said summation network.

3. An injection-locked laser stabilizer as described in claim 1 furtherincluding an amplifier interconnecting said summation network and saidcavity tuner.

1. An injection-locked laser stabilizer comprising a low power stablelaser providing a first beam at a predetermined frequency, a high powerlaser providing a second beam at a preselected frequency, said highpower laser having output means for said second beam and also includinga cavity tuner integrated with said high power laser, audio oscillatormeans providing a preselected frequency signal to modulate said cavitytuner, first means to pass said first beam in a first direction and aportion thereof in a second direction, second means receiving saidpassed first beam for direction into said output means of said highpower laser for injection-locking purposes, said high powerlaser-reflecting said first beam from said output means and alsoproviding therefrom said second beam to said second means, photodetectormeans, third means receiving said reflected first beam and said secondbeam from said second means for direction to said photo-detector means,frequency shifting means, R.F. oscillator means operating at apreselected frequency for driving said frequency shifting means, fourthmeans directing said portion of said first beam to said frequencyshifting means for a preselected frequency shift thus providing a thirdbeam, said third beam also being directed to said photo-detector meansby way of said third means, F.M. discriminator means, said F.M.discriminator having a center frequency equal to the R.F. oscillatorfrequency, A.C. amplifier means, said F.M. discriminator means and saidA.C. amplifier means receiving simultaneously the output signal fromsaid photo-detector means, a product detector receiving a signal fromsaid A.C. amplfier means, a summation network receiving signals fromsaid F.M. discriminator means and said product detector to provide aresultant signal for reception by said cavity tuner to provide anautomatically acquired injection-locked condition and to automaticallymaintain the injection-locked condition in said high power laser.
 2. Aninjection-locked laser stabilizer as described in claim 1 furtherincluding an I.F. amplifier interconnecting said F.M. discriminatormeans and said summation network.
 3. An injection-locked laserstabilizer as described in claim 1 further including an amplifierinterconnecting said summation network and said cavity tuner.